1. Field of the Invention
The present invention relates to an ignition coil of a dual ignition type that is applied to a cylinder having two ignition plugs, and in particular to an ignition coil that has an output terminal mounted on a plug hole of an engine and directly connected to one ignition plug, and an output terminal connected to the other ignition plug in the same cylinder via a high tension cord.
2. Description of the Related Art
In internal combustion engines such as vehicle engines, an ignition coil and an ignition plug are used as, for example, detonators that burn gasoline as fuel.
As an example of an ignition coil, there is known an ignition coil of a dual ignition type that is applied to a cylinder having two ignition plugs.
Examples of publications of prior arts relating to an ignition coil of a dual ignition type, a diagnostic apparatus therefor, or the like include Japanese Examined Patent Publication (Kokoku) No. S63-28238, Japanese Laid-Open Patent Publication (Kokai) No. 2004-247571, and Japanese Laid-Open Utility Model Publication (kokai) No. S59-13853.
In Japanese Examined Patent Publication (Kokoku) No. S63-28238, a diagnostic apparatus that makes a diagnosis as to the floating capacitance and the igniting ability of an ignition coil of a dual ignition type is described.
In Japanese Laid-Open Patent Publication (Kokai) No. 2004-247571, a conventional ignition coil of a dual ignition type is described. FIG. 14 is a connection wiring diagram showing the ignition coil described in Japanese Laid-Open Patent Publication (Kokai) No. 2004-247571. As shown in FIG. 14, a primary coil 281 and secondary coils 282 face each other with a predetermined space left therebetween, and the secondary coils 282 are comprised of two secondary coils 282a and 282b wound in opposite directions (reversely wound) with a center tap 283 being a boundary. The center tap 283, which is the center of the winding width of the secondary coils 282, is connected to one end of the primary coil 281 by wiring 284.
According to this ignition coil of the dual ignition type, outputs of the same polarity are obtained from two secondary output terminals, insulation load in the ignition coil can be reduced, and hence insulation performance can be made stable.
FIG. 15 is a connection wiring diagram showing a conventional ignition coil disclosed in Japanese Laid-Open Utility Model Publication (kokai) No. S59-13853. As shown in FIG. 15, the ignition coil is constructed such that primary coils comprised of a first primary coil and a second primary coil connected in parallel to each other face a secondary coil. That is, the ignition coil is comprised mainly of the primary coils constructed by connecting the first primary coil 291 and the second primary coil 292 in parallel to each other, and the secondary coil 293 disposed such as to face the primary coils with a predetermined space left therebetween.
However, according to the conventional ignition coil of the dual ignition type, outputs from the two secondary output terminals tend to be different due to a difference in the layout of the ignition plugs. Specifically, in the case where one of the secondary output terminals is directly connected to one ignition plug, and the other one of the secondary output terminals is connected to the other ignition plug in the same cylinder via a high tension cord, outputs tend to become unbalanced due to a difference in floating capacitance on wiring extending from the secondary output terminals to the ignition plugs.
Also, the ignition coil of the dual ignition type shown in FIG. 14 has the problem that secondary output in a low-speed range is unlikely to be stable, and the ignition coil shown in FIG. 15 has the problem that interference of the primary coils occurs due to back electromotive force, resulting in output from the secondary coils decreasing. Thus, if the primary coils comprised of the parallel-connected two coils as shown in FIG. 15 are applied to the ignition coil of the dual ignition type, predetermined high-voltage outputs cannot be applied to the two secondary output terminals, and this will bring about a great loss of output from an engine as a whole.